/Engine/Source/Runtime/UMG/Public/Components/Widget.h
Sets the widget navigation rules for a specific direction. This can only be called on widgets that are in a widget tree. This works only for CustomBoundary Rule.
void SetNavigationRuleCustomBoundary ( EUINavigation Direction, [FCustomWidgetNavigationDelegate](API\Runtime\UMG\Blueprint\FCustomWidgetNavigationDelegate) InCustomDelegate )